As described in WO 2012/044964, which is incorporated herein by reference in its entirety, “sensor fusion” is a term covering methods that determine the angular position of a device in an Earth-type reference system based on measurements of a magnetometer and motion sensors attached to the device.
An Earth-type reference system is a frame in which gravity and Earth's magnetic field have fixed known directions. For example, Earth frame, which is defined as having its x-axis pointing north and its z-axis pointing down in gravity's direction (thereby its y-axis pointing east), is an Earth-type reference system.
A sensor reference system is a frame whose axes are defined relative to the rigid body of a device on which the sensors are mounted. Since the device is changing its position and orientation, gravity and Earth's magnetic field do not have fixed and/or known directions in the sensor reference system. The angular position (orientation) of the device in a target (e.g., Earth-type) reference system determines a transformation (i.e., rotations) from the sensor reference system into the target reference system.
Conventionally, sensor measurements are acquired and then filtered in the sensor reference system to remove jitter (insignificant fluctuations) and to separate time-dependent portions from stable portions thereof (e.g., to separate linear acceleration from gravity). That is, since the sensor fusion's goal is to determine a device's orientation and not a device's position, the focus is on extracting time-independent gravity from the accelerometer signal and not on time-dependent linear acceleration other than gravity. However, filtering sensor measurements in the sensor reference system leads to very poor results when the device (and thus the sensor reference system) is moving and changing its angular position.
A superior sensor fusion result would be achieved if the sensor measurements were filtered in the Earth-type reference system, where gravity and magnetic field directions are known. In this case, a filtered accelerometer measurement would match gravity, and a filtered magnetometer measurement would match the Earth's magnetic field. Since portions of the measurements (i.e., linear acceleration from the accelerometer signal and magnetic interference effects from the magnetometer signal) filtered out vary over time and depend on the device's motion, a low-frequency pass filter could be used to remove these portions of the sensor signals transformed in the Earth-type reference system, while keeping the desired (time-invariant or low-frequency varying) portions. However, since the angular position of the sensor reference system with respect to the Earth-type reference system is not known, the measurement cannot be transformed from the sensor reference system in the Earth-type reference system. Therefore, this ideal manner of filtering signals (i.e., in the Earth-type reference system) cannot be implemented.
Accordingly, it is desirable to provide methods for filtering sensor signals in a reference system whose position relative to the sensor reference system is known and in which the advantages of filtering in an Earth-type reference system are preserved.